1. Field of the Invention
This invention is directed to an apparatus and a method of use of such apparatus. More specifically, this invention is concerned with apparatus capable of substantially complete transfer of an electrostatically adherent toner image from a curvilinear surface to an electrostatically charged receiving sheet. The curvilinear surface can correspond to the image recording element or an intermediate transfer element of an electrostatographic copier.
2. Description of the Prior Art
The formation and development of images on the recording surface of electrostatographic copying equipment is well known. The best known of the commercial process, more commonly known as xerography, involves forming a latent electrostatic image on the imaging layer of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing the electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered relatively conductive and electrostatic charge selectively dissipated in these irradiated areas. After this imaging member is exposed, the latent electrostatic image is rendered visible by development with electrostatically charged materials, commonly referred to in the art as "toners". This toner will be principally attracted to those areas of the image bearing surface which retain the electrostatic charge and thus form a visible toner image.
The developed image can then be read or permanently affixed to the imaging layer in the event the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder type photoconductive films (e.g., zinc oxide dispersed in a film forming insulating resin) where the photoconductive insulating layer of the imaging member is also an integral part of the finished copy. In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable imaging member or transferred to another surface, such as a sheet of plain paper and thereafter developed. When the latent image is developed on the imaging surface of a reusable imaging member it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films and solvent or thermal fusion of toner particles to the supportive substrate.
It is essential that during the transfer of the toner image from the surface of the imaging member to the receiving sheet, the receiving sheet remain in contact with the imaging member and be transported in registration therewith. Ordinarily, the receiving sheet is introduced into the above type of reproduction equipment in a manner consistent with the above noted requirements; however, the degree of conformance of the receiving sheet to the imaging member surface will vary depending upon the relative stiffness (flexibility) of materials from which the receiving sheet is formed and the relative thickness of the receiving sheet. As will be readily appreciated, the more arcuate (convex) the imaging surface of the recording element, the greater the difficulty encountered in securing conformance of the receiving sheet thereto. The prior art discloses various techniques for achieving conformance of the receiving sheet to a recording element in the region of electrostatic transfer of toner from the surface of a recording element to a receiving sheet; see for example conformance of a receiving sheet to a photoreceptor by the use of bands (U.S. Pat. No. 3,332,328, FIG. 3, Column 3, lines 52-70) or by means of a biased transfer roll (U.S. Pat. No. 3,702,482). Each of the systems described in the above referenced patents suffer from one or more deficiencies, resulting in the nonuniform transfer of toner particles from the imaging surface of the recording element to the receiving sheet. In the transfer system described in the U.S. Pat. No. 3,332,328 the relatively broad bands can shield the backside of the transfer sheet from corona emission, resulting in variation in the fields placed across the transfer sheet and thus the non-uniform transfer of toner from the recording element to the receiving sheet. The transfer system disclosed in the U.S. Pat. No. 3,702,482 is similarly deficient in that it is highly humidity sensitive. Experience has shown that in moist environments the transfer roller will not satisfactorily affect transfer of the toner image from the recording element to the transfer sheet.
Accordingly, it is the object of this invention to remedy the above as well as related deficiencies in the prior art.
More specifically, it is the primary object of this invention to provide apparatus which will permit the substantially uniform transfer of an electrostatically adherent toner image to a receiving sheet from a toner bearing surface of an electrostatographic copier.
It is another object of this invention to provide apparatus which is capable of effecting substantially uniform transfer of an electrostatically adherent toner image to a relatively inflexible receiving sheet from a photosensitive recording element of an electrostatic copier.
It is yet another object of this invention to provide apparatus suitable for the electrostatic transfer of electrostatically adherent toner particles to a relatively inflexible receiving sheet from the photosensitive element of an electrostatographic copier in a manner which is intended to minimize abrasive contact with the imaging surface of the photosensitive element of said copier.
Still yet another object of this invention is to provide apparatus wherein the transfer assembly is combined with additional means capable of vacuum assisted stripping of the receiving sheet from an image bearing surface subsequent to toner transfer to the receiving sheet.
Additional objects of this invention include the combination of the transfer apparatus of this invention with other components of electrostatographic copying equipment and the use of such combinations in electrostatographic recording methods.